Advances in polymer chemistry and technology over the last few decades have enabled the development of high-performance polymeric fibers. For example, liquid-crystalline polymer solutions of heterocyclic rigid-rod polymers can be formed into high strength fibers by spinning liquid-crystalline solutions into wet fibers, removing solvent to dry the fibers, and heat treating the dried fibers. Examples of high-performance polymeric fibers that include poly(p-phenylene benzobisthiazole) (“PBZT”) and poly(p-phenylene-2,6-benzobisoxazole) (“PBO”).
Fiber strength is typically correlated to one or more polymer parameters, including composition, molecular weight, intermolecular interactions, backbone, residual solvent or water, macromolecular orientation, and process history. For example, fiber strength typically increases with polymer length (i.e., molecular weight), polymer orientation, and the presence of strong attractive intermolecular interactions. As high molecular weight rigid-rod polymers are useful for forming polymer solutions (“dopes”) from which fibers can be spun, increasing molecular weight typically results in increased fiber strength.
Molecular weights of rigid-rod polymers are typically monitored by, and correlated to, one or more dilute solution viscosity measurements. Accordingly, dilute solution measurements of the relative viscosity (“Vrel” or “ηrel” or “nrel”) and inherent viscosity (“Vinh” or “ηinh” or “ninh”) are typically used for monitoring polymer molecular weight. The relative and inherent viscosities of dilute polymer solutions are related according to the expressionVinh=ln(Vrel)/C, where ln is the natural logarithm function and C is the concentration of the polymer solution. Vrel is a unitless ratio, thus Vinh is expressed in units of inverse concentration, typically as deciliters per gram (“dl/g”).
Rigid-rod polymer fibers having strong hydrogen bonds between polymer chains, e.g. polypyridobisimidazoles, have been described in U.S. Pat. No. 5,674,969 to Sikkema et al. An example of a polypyridobisimidazole includes poly(1,4-(2,5-dihydroxy) phenylene-2,6-pyrido[2,3-d:5,6-d′]bisimidazole), which can be prepared by the condensation polymerization of 2,3,5,6-tetraminopyridine and 2,5-dihydroxyterephthalic acid in polyphosphoric acid. Sikkema describes that in making one- or two-dimensional objects, such as fibers, films, tapes, and the like, it is desired that polypyridobisimidazoles have a high molecular weight corresponding to a relative viscosity (“Vrel” or “ηrel”) of at least about 3.5, preferably at least about 5, and more particularly equal to or higher than about 10, when measured at a polymer concentration of 0.25 g/dl in methane sulfonic acid at 25° C. Sikkema also discloses that very good fiber spinning results are obtained with poly[pyridobisimidazole-2,6-diyl(2,5-dihydroxy-p-phenylene)] having relative viscosities greater than about 12, and that relative viscosities of over 50 (corresponding to inherent viscosities greater than about 15.6 dl/g) can be achieved. Accordingly, further technical advances are needed to provide even higher molecular weight rigid-rod polymers, such as polypyridobisimidazoles, that are characterized as providing polymer solutions having even greater viscosities and producing fibers from such polymer solutions.